The purpose of this research is to identify the mechanisms responsible for the accumulation of fluid inside the expanding rabbit blastocyst. There is good suggestive evidence that active ionic transport is involved in blastocyst expansion. We will determine which ion(s) are being actively transposed across the blastocoele wall by measuring, simultaneously, the short-circuit current and net isotope flux (Na22, Cl36, K42) across the perfused rabbit blastocyst. Agents, such as hormones, drugs, gases, etc., will then be tested for their effects on the electrical and transport properties of the blastocyst. Those agents that affect the specific active ionic transport processes involved will be utilized to study the relationship between active ionic transport and expansion (water movement).